MOSCOW?Militants stormed a hydroelectric plant in Russia's Caucasus region in a brazen dawn attack Wednesday, killing two guards and setting the facility ablaze with a string of blasts, officials said.

The unknown attackers, who also beat up plant employees, set off the explosions at the station in the volatile North Caucasus's Kabardino-Balkaria region by laying mines in the turbine room. The plant was shut down as a result.

"A terror act took place at the Baksanskaya hydroelectric power plant," state-run power group RusHydro, which runs the plant, said in a statement.

The authorities are battling a Muslim insurgency in the Caucasus, where Moscow fought two bloody wars against Chechen separatists in the 1990s and militants have long pledged to destroy key infrastructure sites.

President Dmitry Medvedev said in a message on Twitter: "Spoke to head of FSB (security service) and Kabardino-Balkarian president. Security at strategic sites tightened after today's explosions at the HPP."

Prime Minister Vladimir Putin was in contact with Igor Sechin, his powerful deputy in charge of the energy sector, and told him to make sure the incident did not disrupt energy flow in the region, Putin's spokesman Dmitry Peskov told AFP.

Regional police said a group of between three and five attackers burst onto the premises of the station in the village of Atazhukino and killed two police guards and stole their weapons.

"The assailants mined the power plant's turbine room," police said, adding that two turbines were blown up, setting the engine room on fire. The blaze took several hours to put out because of boiling oil, officials said.

Investigators said two explosions shook the plant's turbine room while another two hit the transformer vault. One more explosive device was found and neutralized by sappers at the scene.

The visibly shaken director of the hydroelectric plant, Valery Shogenov, said on television that employees had been tied up with duct tape but managed to release themselves.

Regional RusHydro spokesman Alim Balkizov said staff were tortured and beaten up, with one also receiving knife cuts. Two received medical help and were recovering, he added.

A regional police spokesman told AFP "the assailants or their accomplices" also attacked a police building in the nearby town of Baksan, possibly to deflect attention from the attack on the hydroelectric plant.

Authorities said a criminal case was opened into the explosions but refused to say who exactly may be behind the attack. Officials said the plant had been halted and there was no risk of a flood.

Security experts said the attack underscored how vulnerable the country's infrastructure was to militant attacks.

"It laid bare the vulnerability of the security system in the Caucasus," Andrei Soldatov, editor of Agentura.ru website with sources in the intelligence agencies, told AFP.

Some said they feared the attack on the relatively small, 25-megawatt power plant built in the 1930s might be a precursor to more disastrous events.

"I fear it might be a rehearsal of something more large-scale," senator Alexander Torshin said in comments on Echo of Moscow radio.

The incident is the latest blow to RusHydro. Last August, 75 people died in an accident at the Sayano-Shushenskaya dam, a Siberian hydropower station that the state-run power group operates.

That incident was claimed by an Islamist group but authorities insisted technical faults were to blame.

In March, Russia was shaken to the core when two female suicide bombers, both from the North Caucasus region of Dagestan, killed 40 people in a pair of coordinated attacks on the Moscow metro.

Copyright 2015 Agence France-Presse. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.